There exists an on-going effort in the automotive vehicle community to reduce the engine and powertrain weight in an effort to improve fuel economy. As a key part of this effort, many engine structural components that were traditionally made from relatively heavy materials such as steel and cast iron are now being made from lighter metals. One of these lighter materials is aluminum which is about one-third the weight of a comparable component made from iron. Other lightweight metals, such as magnesium, have also been substituted for the heavier steel and cast iron. Magnesium is itself about two-thirds the weight of aluminum.
While these lighter metals readily demonstrate their weight advantage over steel and cast iron, these metals, and particularly magnesium, are difficult and expensive to produce. In addition, these metals, again particularly magnesium, can fail at attachment points. Further in the case of magnesium, this material is susceptible to mismatches of thermal expansion coefficients which presents a problem when different materials are attached to one another. Furthermore, many magnesium alloys exhibit unacceptable levels of a phenomenon known as “creep” when placed under thermal load in internal combustion engine applications where high operating temperatures are common. The result of thermal creep can be both a reduction of clamping force as well as an increased possibility of fastener loosening at the point of attachment. Accordingly, the use of magnesium has greater challenges than, for example, the use of aluminum as a substitute for steel and cast iron in the manufacture of engine components.
In response to the problems associated with the use of lightweight metals such as aluminum and magnesium in the production of associated engine components, some automotive manufacturers have moved away from using metals for these components altogether. Instead, some manufacturers have used any one of several polymerized materials for these components. A variety of materials, including reinforced plastic materials such as glass-filled nylon or glass-filled or carbon-reinforced polypropylene, have been used for the production of engine components.
While providing an attractive weight advantage over steel and cast iron and providing lower cost and easier manufacturing than part production using either aluminum or magnesium, engine components made from plastic composites also suffer from the problem of creep associated with parts made from magnesium. Over time, a bolt used for attaching a composite part to a substrate may eventually back out of the composite material as the area around the bolt creeps outwardly from under the bolt. This is typically the result of the inherent vibration of the internal combustion engine and this situation becomes more apparent the longer the engine is in use.
Fasteners of several designs have been utilized to fasten one plastic engine component to another in an effort to overcome the above-described in-use challenges. Such fasteners must be suitably designed to prevent damage to the relatively brittle plastic components. An example of such a fastener is a spring-stem fastener that has been used to threadably attach a first plastic engine component to a second plastic engine component. However, tolerances associated with the resulting joint cause conditions where there can be either over-compression of the fastener spring at one extreme and under-compression or a complete absence of compression of the fastener spring at the other extreme.
As in so many areas of vehicle technology there is always room for improvement related to arrangements for attaching plastic components of a vehicle engine.